Inductive coupling of a data signal for a power transmission cable

ABSTRACT

There is provided a coupler for a data signal. The coupler includes a core through which a phase conductor of a power distribution system can be routed, and a winding wound around a portion of the core. The data signal is inductively coupled between the phase conductor and the winding via the core.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 09/948,895, file Sep. 7, 2001, U.S. Pat No. 6,646,447 which isa divisional of U.S. patent application Ser. No. 09/752,705, filed onDec. 28, 2000, now U.S. Pat. No. 6,452,482, which claimed priority of(a) U.S. Provisional Patent Application Ser. No. 60/1 73,808, filed onDec. 30, 1999, and (b) U.S. Provisional Patent Application Ser. No.60/198,671, filed on Apr. 20, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication of a data signal over apower distribution system, and more particularly, to a use of aninductive coupler for coupling a data signal via a conductor in a powertransmission cable.

2. Description of the Prior Art

Low voltage (LV) power lines within the confines of a home or businesshave been used as a medium for point to point or network communicationsusing so called “carrier” systems in which a data signal is modulatedonto a high frequency (HF) carrier and transmitted over the power lines.Internet access, which requires “last mile” connectivity between theInternet data trunk and each domicile, would greatly enhance the utilityof such networks.

A medium voltage (MV) typically 4-66 kV is reduced to a low voltage (LV)typically 100-500 volts, through an MV-LV distribution transformer. Amedium voltage power distribution grid feeds many homes and businessesvia distribution transformers. If data is present on the medium voltagepower grid, it would be desirable to couple broadband data streams fromtransformer substations to entire sections of a neighborhood, but thedistribution transformers effectively block high frequency energy andthus block the data from getting to the LV drop lines.

In countries using nominal low voltages of 125 volts or less, such as inNorth America, drop lines from the distribution transformer to theelectrical load in the home or business are usually kept shorter thanabout 50 meters, so as to minimize voltage drop across the lines and topreserve adequate voltage regulation. Typically, only one to ten homesor businesses are supplied from each distribution transformer. For sucha small number of potential users, it is not economical to procure anexpensive high data rate feed, such as fiber or T1, and couple it viapower line communications devices to the low voltage side of thetransformer. Accordingly, in order to exploit the medium voltagedistribution grid as a data backhaul channel, a device is required tobypass the distribution transformer.

In a power distribution system, a high voltage (HV) typically 100-800kV, is stepped-down to a medium voltage through an HV-MV step-downtransformer at a transformer substation. The high frequency blockingcharacteristics of distribution transformers isolate the medium voltagepower distribution grid from high frequency noise present on both thelow voltage and the high voltage (HV) lines. The medium voltage grid isthus a relatively quiet medium, ideal for communicating high speed dataas a data distribution system or “backhaul line.”

The above-mentioned transformers block practically all energy in themegahertz frequency range. In order to couple high frequency modulateddata from the MV lines to the LV lines, a bypass device must beinstalled at each transformer site. Devices are presently available andused for low frequency, low data rate data coupling applications. Suchapplications are often termed Power Line Communications (PLC). Thesedevices typically include a high voltage series coupling capacitor,which must withstand a Basic Impulse Loading (BIL) voltage, typicallyabove 50 kV. Such devices are thus expensive, bulky, and have an impacton overall power grid reliability. Furthermore, in some cases, duringtheir installation they require disconnecting power from the customers.

In countries having a nominal low voltage in the 100-120 volt range,such as Japan and the US, the number of distribution transformers isespecially large. This is because the MV-LV distribution transformersare placed relatively close to the load to keep the feed resistance low.Low feed resistance is desired to maintain reasonable level of voltageregulation, that is, minimal variation in supply voltage with varyingload currents. LV feed lines for distances much in excess of 50 meterswould require impracticably thick wires.

For a data coupler to be effective, it must be considered in the contextin which it operates in conjunction with the high frequencycharacteristics of the MV power lines and with other componentsconnected to these lines, such as transformers, power factor correctioncapacitors, PLC coupling capacitors, and disconnect switches. Thesecomponents operate at different voltages in different countries andregions. The operating voltage level has a direct impact on the geometryof the construction of medium voltage power devices and the terminalimpedance of these devices at Megahertz frequencies. Other factorsaffecting high frequency signals on MV power lines include the geometryof the network, e.g., branching, the use of very low impedanceunderground cables that connect to high impedance overhead lines, andthe possibility of a splitting of a network into sub-networks due to anactuation of a disconnect switch. Therefore, the suitability of an MV-LVcoupler device must be considered in the context of the specificcharacteristics of the equipment used in each country and the MV voltagelevel.

Overhead transmission lines are characterized by two or mores wires runat essentially constant spacing, with air dielectric between them. Suchlines have a characteristic impedance in the 300 to 500 ohms range, andvery low loss. Coaxial underground cables comprise a center conductorsurrounded by a dielectric, over which are wound neutral conductors.Such cables have a characteristic impedance in the range of 20 to 40ohms, and display loss for Megahertz signals that may be as low as 2 dBper hundred-meter length, depending on the loss properties of thedielectric.

An MV-LV distribution transformer, whether designed for operation fromsingle phase to neutral or from phase to phase in a three phase grid,has a primary winding on the MV side that appears as having an impedancein the 40 to 300 ohm range for frequencies above 10 MHz. Power factorcorrection capacitors have large nominal capacitance values (e.g. 0.05-1uF), but their high frequency impedance is primarily determined byseries inductance inherent in their construction. PLC couplingcapacitors have lower nominal capacitances, for example, 2.2-10 nF, butmay have high frequency impedances that are relatively low relative tothe power cable's characteristic impedance. Any of the aforementioneddevices may produce a resonance in the megahertz range, i.e., theimaginary part of a complex impedance becomes zero ohms, but the devicesdo not have high Q factors at these frequencies, and so the magnitude ofthe impedance typically does not approach zero for a series resonance oran extremely high value for a parallel resonance.

Another device used on MV grids, especially in Japan, is a remotelycontrolled three phase disconnect switch. When a data signal istransmitted over a phase line that passes through such a switch,continuity of the data needs to be maintained even when the phase lineis opened through the switch.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved couplerfor coupling a data signal to a conductor in a power transmission cable.

It is another object of the present invention to provide such a couplerthat is inexpensive and has a high data rate capacity.

It is a further object of the present invention to provide such acoupler that can be installed without interrupting service to powercustomers.

It is still a further object of the present invention to provide such acoupler using only passive components that have a virtually unlimitedservice life.

These and other objects of the present invention are achieved by acoupler for a data signal. The coupler includes a core through which aphase conductor of a power distribution system can be routed, and awinding wound around a portion of the core. The data signal isinductively coupled between the phase conductor and the winding via thecore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical underground coaxial mediumvoltage distribution cable, showing a neutral wire being used as acommunication medium, in accordance with the present invention.

FIG. 2A is an illustration of an arrangement of a single-endedtransmission line using a single neutral wire for data communication, inaccordance with the present invention.

FIG. 2B is a schematic representation of the arrangement of FIG. 2A.

FIG. 3A is an illustration of a power transmission cable in which twoneutral wires are used as a transmission line for communication of adata signal, in accordance with the present invention.

FIG. 3B is a schematic representation of the arrangement shown in FIG.3A.

FIG. 3C is a schematic representation of an alternative to thearrangement shown in FIG. 3A using a plurality of neutral wires to forma data transmission line.

FIG. 3D is an illustration of a technique for implementing thearrangement shown in FIG. 3C.

FIGS. 4A and 4B illustrate embodiments of a magnetic core topology for acoupler for use with a pair of neutral conductors that aredifferentially driven with a data signal.

FIG. 5A is an illustration of an arrangement of a cable having a highimpedance introduced by placement of a magnetic toroid core.

FIG. 5B is a schematic representation of the arrangement of FIG. 5A.

FIGS. 6A-6C are illustrations of several arrangements of a balancedtransmission line using two neutral wires and magnetic induction, inaccordance with the present invention.

FIG. 6D is a schematic representation of the arrangements of 6A-6C.

FIG. 7 is a schematic of a balanced transmission line using a magneticinduction, in accordance with the present invention.

FIG. 8 is a schematic of an embodiment of the present invention usingmultiple transmission lines with multiple sets of neutral wires.

FIG. 9A is a schematic of a system for identifying one of a plurality ofwires of a power transmission cable.

FIG. 9B is an illustration of a system for identifying one of aplurality of wires of a power transmission cable.

FIGS. 10A and 10B are schematics of portions of a data communicationnetwork implemented over a power distribution system, where the data iscarried on a phase conductor of the power distribution system, inaccordance with the present invention.

FIG. 11A is an illustration of an embodiment of an inductive coupler forcoupling data via a phase conductor, in accordance with the presentinvention.

FIG. 11B is a schematic representation of the embodiment shown in FIG.11A.

FIG. 12 is a schematic of a portion of a network having back-to-backmodems at an inductive coupler.

FIG. 13 is a schematic of a technique for passively coupling modulateddata between segments of a power grid, in accordance with the presentinvention.

FIG. 14 is a schematic of a technique for coupling modulated databetween segments of a power grid using back-to-back modems.

FIG. 15 is a schematic showing several techniques for coupling data to aphase conductor of a power distribution system in an implementation of adata communication network, in accordance with the present invention.

FIG. 16A is a schematic for a capacitive coupler for terminating atransmission line dead end, in accordance with the present invention.

FIG. 16B is a schematic using a capacitive coupler for connecting amodem to a transmission line dead end, in accordance with the presentinvention.

FIG. 16C is a schematic of an arrangement of a capacitive coupler formaintaining continuity of a data signal across a grid disconnect switch,in accordance with the present invention.

DESCRIPTION OF THE INVENTION

Overhead and underground medium voltage transmission lines may be usedfor the bi-directional transmission of digital data. Such transmissionlines cover the path between a power company's transformer substationand one or more MV-LV distribution transformers placed throughout aneighborhood. The MV-LV distribution transformers step the mediumvoltage power down to low voltage, which is then fed to homes andbusinesses.

The present invention relates to a use of a coupler in a medium voltagegrid. The coupler is for enabling communication of a data signal via apower transmission cable. It has a first winding for coupling the datasignal via a conductor of the power transmission cable, and a secondwinding, inductively coupled to the first winding, for coupling the datasignal via a data port.

One embodiment of the present invention is employed with a powertransmission cable having one or more neutral wires, i.e., conductors,wrapped around an outer layer of the cable, similarly to a coaxialcable. One or more of the neutral wires of the power transmission cableserves as a conductor for one or more data signals.

Another embodiment is employed with a phase conductor of a powertransmission cable. In this case, the phase conductor of the powertransmission cable serves as a conductor for one or more data signals.

FIG. 1 is an illustration of a typical underground coaxial mediumvoltage distribution cable 100 with an inductive coupler coupledthereto, in accordance with the present invention. Cable 100 has amultiplicity of N neutral conductors 105 wrapped spirally around a coreinsulator 120, which surrounds a phase conducting wire 115. For example,in a Pirelli Cable X-0802/4202/0692 TRXLPE 25 KV 260 mils 1/0 A WG A1cable, which is available from Pirelli Cavi e Sistemi S.p.A., VialeSarca, 222, Milano, Italy 20126, there is a phase conducting wiresurrounded by insulation around which are wound 8 strands of 2.8 mmdiameter copper. Cables having 12 to 16 neutral conductors are alsocommon.

Neutral conductors 105 are separated and insulated from each other in acable segment. At an end of cable 100, a strand of each neutralconductor 105 is exposed and wrapped tangentially, forming a ring ofcopper wire 125 a short distance from the end of the cable, to form aterminus. These strands are gathered together into a single strandedwire 130 and connected to a grounding post at an MV-LV distributiontransformer.

A coupler 140 is already insulated from phase conductor 115, the lattercertified to withstand both the steady state and transient voltages forwhich the cable is rated. Exploiting existing insulation obviates theexpense of providing it again for the coupler. The coupler can bepackaged with ordinary plastic materials.

Coupler 140 includes a first winding (not shown in FIG. 1) and secondwinding (not shown in FIG. 1). The first winding is provided by thecable itself while the second winding can comprise one or two turns ofstranded, small diameter hookup wire, with minimal insulation.

In an underground cable, such as cable 100, the use of inductive coupler140 is particularly cost effective, as it takes advantage of theexisting insulator 120 to provide isolation from the medium voltagelines.

An inductive coupler in accordance with the present invention is alsosuitable for use with an overhead power transmission cable. Theinductive coupler is generally less expensive than a capacitive coupler,as increasing the thickness of the inductive coupler's insulation doesnot substantially degrade the coupler performance, while increasing theinsulation thickness in the capacitor directly decreases its capacitanceper unit area, and necessitates a larger plate area. Therefore, incomparison to a capacitive coupler, the inductive coupler isconsiderably less expensive to manufacture.

There are several alternative embodiments of the invention. Forunderground cable, one can enlist one or more of the neutral wires ofthe underground cable, which can form high frequency transmission lines,while the power conduction function of the selected neutral wire(s) ispreserved.

FIG. 2A is an illustration of an arrangement of a single-endedtransmission line using a single neutral wire for data communication, inaccordance with the present invention. FIG. 2B is a schematicrepresentation of the arrangement of FIG. 2A. A cable 200 includes amultiplicity of neutral conductors 205, e.g. wires, that can beconsidered as a flat data transmission line, wrapped in a gentle spiralaround a high voltage insulator 240 and a center phase conductor 245.

One selected strand of neutral conductors 205, i.e., neutral conductor202, is isolated to act as a data transmission line conductor for a datasignal, and the remaining neutral conductors 205, mainly two neutralconductors 205 that are adjacent to neutral conductor 202, serve as asecond data transmission line conductor. For the cross section of thePirelli cable-described above, the characteristic impedance is estimatedto be about 95 ohms with respect to signals in a frequency range of 1-50MHz, a subrange of which is typically used in a transmission of data.

To implement the arrangement of FIG. 2A in an already-installedunderground cable, neutral conductor 202 is selected out of the severalneutral conductors 205, and cut in an exposed section 210 at each end ofcable 200. A lead 215 of neutral conductor 202 remains connected to aring 250 at each end of cable 200. Neutral conductor 202 and lead 215are connected to a first winding 225 of a coupler 220. First winding 225is thus connected in series between neutral conductor 202 and ground. Asecond winding 235 of coupler 220 is coupled to a port 255 through whichdata is transmitted and received. Thus, cable 200 is enlisted for use asa high frequency transmission line, which can be connected tocommunications equipment such as a modem (not shown), via coupler 220.

Electrically speaking, coupler 220 is a transformer. The impedanceacross the primary, i.e., first winding 225, of such a transformer isnegligible at the frequencies used for conducting power. First winding225, which is attached to neutral conductor 202 and lead 215, should bewound with a wire at least as thick as that of neutral conductor 202.Under these circumstances, the selected data-carrying neutral conductor202 has essentially the same impedance as all of the other neutralwires. It would carry essentially the same current as each of the otherneutral wires, and the total ampacity and surge current capacity of theneutral circuit would not be degraded.

In FIGS. 2A and 2B, the neutral current of the single neutral conductor202 passes through coupler 220. For a 200 Amp cable with eight neutralwires, the data-carrying wire would carry a maximum steady state currentof 25 Amps rms. The maximum steady-state current through a singleneutral conductor is less for a smaller ampacity cable and for a cablewith a larger number of neutral conductors. Coupler 220 must be capableof handling the flux generated by this current, without magnetic coresaturation, in order to carry out its data coupling function.

Neutral conductor 202 carries current in a first direction for a highfrequency data signal. The other neutral conductors 205 carry the datasignal's return current in the opposite direction, tending to cancel andthus greatly decrease an intensity of the radiated magnetic field due tothe modulated data signal. This arrangement also provides anelectrostatic shielding effect against noise coupling from an externalelectric field.

FIG. 3A is an illustration of a power transmission cable 300 in whichtwo neutral wires are used as a transmission line for communication of adata signal, in accordance with the present invention. FIG. 3B is aschematic representation of the arrangement shown in FIG. 3A.

A coupler 307, for example, a high frequency transformer, is installedin series with two adjacent neutral wires 302, 305. Neutral wires 302,305, which are preferably in parallel and adjacent to one another, arecut just before a point where they attach to neutral connecting ring330.

Referring to FIG. 3B, the leads of neutral wires 302, 305 extending fromcable 300 are connected to a first winding 310 of coupler 307. Firstwinding 310 is thus connected in series between neutral conductor 302and neutral conductor 305. First winding 310 includes a center-tap 312and a magnetic core 315. Center-tap 312 is connected to neutralconnecting ring 330.

A portion 310A of first winding 310 is connected to neutral wire 302 andwound in a first direction around core 315, and a second portion 310B offirst winding 310 is connected to neutral wire 305 and wound in theopposite direction around core 315. Portions 310A and 310B are made ofwires of slightly larger diameter than the power cable neutral wires,and are therefore capable of carrying steady state and surge currents atleast as well as the unselected neutral conductors. Each of portions310A and 310B may itself be considered a winding.

The arrangement of FIG. 3A ensures that only a negligible impedance isinserted in series with two neutral wires 302, 305, and does not disturbthe essentially equal division of power frequency current among all ofthe neutral wires. For the Pirelli cable described earlier, thecharacteristic impedance of the parallel wires 302 and 305 acting as aparallel wire transmission line is estimated to be approximately 130ohms. Also, at power frequency, the arrangement shown in FIGS. 3A and 3Bresults in flux cancellation due the neutral currents' flowing inopposite directions in windings 310A and 310B, resulting in a negligiblenet flux through core 315.

Another winding 320 is connected to a port 350 through which data istransmitted and received. Winding 320 is insulated from the powercircuit neutral 325, thus avoiding a ground loop that could inducespurious noise and fault surges into the data circuits.

Cable 300 can be thought of as a high frequency transmission line, whichcan be connected to communications equipment via coupler 307. In thisconfiguration, a data signal is driven differentially through neutralconductors 302, 305. Such a transmission line should emit even lowerelectromagnetic radiation than the singe-ended arrangement described inFIG. 2A, for a given drive power level.

FIG. 3C is a schematic representation of an alternative to thearrangement shown in FIGS. 3A and 3B using a plurality of neutral wiresto form a data transmission line. Cable 300 has a plurality of neutralwires 330 that are substantially parallel to one another, withindividuals of a first subset 330A of the plurality of neutral wires 330alternating with individuals of a second subset 330B of the plurality ofneutral wires 330. The first subset 330A is collectively regarded as afirst neutral conductor and joined together to form a first strandedlead 332 to a coupler 307A. The second subset 330B is collectivelyregarded a second neutral conductor and joined together to form a secondstranded lead 333 to coupler 307A. Preferably, the plurality of neutralwires 330 is configured as N/2 transmission lines connected in parallel,where N is the number of neutral wires 330, and N/2 is the number ofneutral wires in each of subsets 330A and 330B. The effect of such aparallel connection is to reduce the attenuation produced by cable 300by a factor of approximately N/2, and to lower the characteristicimpedance by the same factor.

FIG. 3D is a diagram showing how the arrangement of FIG. 3C may beconveniently implemented. To facilitate the attachment of first subset330A to first stranded lead 332, a first insulating ring 335 is placedover all neutral conductors, i.e., first subset 330A and second subset330B, proximate to a point where coupler 307A will be located. Firstsubset 330A is wrapped over first insulating ring 335 and joinedtogether to form first stranded lead 332. Likewise, second subset 330Bis wrapped over a second ring 345, which may be insulating or notinsulating, and joined together to form second stranded lead 333. Theimproved geometrical symmetry of the current flow and reduced voltagelevels should further reduce electromagnetic radiation, relative to thatemitted with the two-wire implementation of FIG. 3A.

An electric utility company might object to cutting two neutral wiresand reconnecting them through a coupler. In accordance with the presentinvention, it is possible to “wind” a magnetic core around the twoselected neutral wires in a manner that is topologically andmagnetically equivalent to the embodiment shown in FIGS. 3A and 3B.

FIGS. 4A and 4B illustrate embodiments of a magnetic core topology for acoupler for use with a pair of neutral conductors that aredifferentially driven with a data signal. Such a core has a first regionadjacent to a first neutral conductor, and a second region adjacent to asecond neutral conductor. The coupler includes a winding wound around aportion of the core. Through the core, the winding induces a firstcurrent in the first neutral conductor in a first direction, and inducesa second current in the second neutral conductor in a second directionthat is opposite of the first direction.

Referring to FIG. 4A, a core 400 may be visualized as a figure “8”, withno contact at the crossing point. The figure “8” forms a topological“twist”. A first region comprises a first loop 405 of the figure “8”. Afirst neutral conductor 410 is routed through first loop 405. A secondregion comprises a second loop 415 of the figure “8”. A second neutralconductor 420 is routed through second loop 415. Core 400 is effectivelya contiguous one-window core through which conductors 410 and 420 arepassed in opposite directions, thus canceling the flux due to powerfrequency currents. A winding 425 induces oppositely-phased highfrequency signal currents in neutral wires 410 and 420.

The figure “8” topology can be implemented on the surface of a cable,without cutting the neutral conductors. As shown in FIG. 4B, a corecomprising core segments 400A and 400B is configured with a first gap430 in the first loop 405 and a second gap 435 in the second loop 415.Neutral conductor 410 is routed through first gap 430 and neutralconductor 420 is routed through second gap 435. By placing cores 400Aand 400B against the insulation 440 of neutral conductors 410 and 420,neutral conductors 410 and 420 are placed within the path of magneticflux.

Another method for avoiding the physical cutting of the neutral wires isto insert a high impedance for high frequencies in series with themwithout cutting the wires. The present invention accomplishes this bysurrounding the entire cable with one or more magnetic toroid cores.

FIG. 5A is an illustration of an arrangement of a cable having a highfrequency high impedance introduced by placement of a magnetic toroidcore over the cable. FIG. 5B is a schematic representation of thearrangement of FIG. 5A.

One or more magnetic toroid cores 502 are disposed around a portion of apower transmission cable 500. A first winding 530 (FIG. 5B) of a coupler515 is connected between a first neutral conductor 510 and a secondneutral conductor 512, inwards of cable 500 relative to magnetic toroidcores 502. A second winding 532 of coupler 515 provides a data path to amodem port 520.

First and second neutral conductors 510, 512 are two of a plurality ofneutral conductors 505 within cable 500. Each of neutral conductors 505will effectively see a choke 502A (FIG. 5B) just prior to a neutralcollection ring 525. Thus, magnetic toroid cores 502 insert an isolatingreactance between each of neutral wires 505 and ground, preferably onthe order of magnitude of a few micro-Henries.

Magnetic toroid cores 502 may be configured as a split core of twohalves, with a mechanical package provided to mate the core halvesaccurately, and fasten the core to cable 500. The advantage of thisembodiment is that none of the neutral wires 505 need be cut duringinstallation of magnetic toroid cores 502.

A data signal can be transmitted to and received from a modem (notshown) connected across a port 520 of coupler 515 and coupled to neutralconductors 510, 512 upstream of magnetic toroid cores 502. Cable 500 canbe thought of as a high frequency transmission line with connection endpoints 535 and 540 partially isolated from ground by toroids acting aschokes.

At power frequency, the net current passing through the magnetic toroidcores 502 is essentially zero, since the phase current of a centralconductor 517 flowing in one direction is balanced by the oppositelydirected neutral current flowing through the multiplicity of neutralwires 505, all passing through magnetic toroid cores 502. Coresaturation is thus obviated. Power current distribution among theneutral wires 505 is remain unchanged by the presence of magnetic toroidcores 502, as a very small reactance is induced by the choking effect ofmagnetic toroid cores 502, which affect all neutral wires equally.

FIGS. 6A-6C are illustrations of several arrangements of a balancedtransmission line using two neutral wires and magnetic induction, inaccordance with the present invention. FIG. 6D is a schematicrepresentation of the arrangements of 6A-6C. Again, the advantageobtained is the avoidance of cutting or manipulating the neutral wires,for circuits that may or may not be energized.

Each of the embodiments of FIGS. 6A-6D uses two neutral wires as atransmission line. Signal current is magnetically induced in thesections of the neutral wires, adjacent to a grounded collection ring.An open magnetic core (such as an “E” core) is positioned proximate andperpendicular to the two neutral wires.

As shown in FIG. 6A, an open magnetic core 605 has a first leg 606positioned proximate and perpendicular to a first one of two neutralwires 602 of a cable 600, a second leg 607 positioned proximate andperpendicular to a second one of neutral wires 602, and a third leg,i.e., common leg 610, located between first leg 606 and second leg 607.Common leg 610 has a winding 608 wound thereabout.

Winding 608 is wound around common leg 610, which is positioned betweenthe two neutral wires 602 of cable 600. This arrangement inducescurrents in the individuals of neutral wires 602 in opposite directionsfrom each other. A segment 615 (FIG. 6B) of neutral wires 602terminating together in a grounded collection ring 625 (FIG. 6B) mayalternatively be considered a one turn coil passing through the gapbetween the pole faces of legs 606 and 610, and between the pole facesof legs 607 and 610. Thus, a signal current in winding 608 will inducesignal current in the two neutral wires 602, launching a differentialsignal down the transmission line formed by those two neutral wires 602.

Referring to FIG. 6C, to reduce the size of the relatively large air gapbetween the legs in standard core shapes (e.g., “E” core), and toincrease the coupling coefficient, a pair of magnetic toroidal cores 620can be used, with gaps 627 provided through which neutral wires 602 arerouted. A winding 630 is wound around a portion of each of magnetictoroidal cores 620, e.g., a common leg 632.

The equivalent circuit of the embodiments of FIGS. 6A-6C is shown inFIG. 6D. The sections of neutral wires 602 in which the flux is inducedact as two oppositely phased windings 635 connected together atcollection ring 625. A winding 645 provides a port 640 for a connectionto a modem (not shown).

Power frequency magnetomotive force (MMF) is canceled in the common legof the core, but appears in full on each side leg. However, the air gap,which must be larger than the diameter of a neutral wire, would usuallyprevent these side legs from becoming saturated.

The advantage of the embodiments of FIGS. 6A-6D is an avoidance of bothinterruption and physical contact with the neutral wires 602. Currentdistribution among the neutral wires at power frequency would remainessentially unchanged, as the very small reactance induced by the core'schoking affect would introduce a negligible reactance compared to theoverall neutral wire impedance over the entire cable segment. Cable 600can be thought of as a high frequency transmission line, connected ateach terminus via a coupler, to communications equipment.

FIG. 7 is a schematic of a balanced transmission line using a magneticinduction, in accordance with the present invention. This embodiment issimilar to that of FIG. 6D, but instead of a single magnetic core orpair of toroids coupling to one pair of neutral wires, it couples to allneutral wires, organized as pairs. For a cable with an odd number ofwires, one wire would be left-unused. To achieve this, any of theembodiments of FIGS. 6A-6D may be employed, with the number of couplersequaling the number of neutral wire pairs, and the windings of thecouplers connected together. For minimum radiation, alternate neutralwires should be oppositely phased.

Similar to the embodiments of FIGS. 6A-6D, the embodiment of FIG. 7includes a coupler having a first winding 720 for coupling a data signalvia a first neutral conductors 702 of a power transmission cable 700 anda second winding 740, inductively coupled to first winding 720, forcoupling the data signal via a data port 760. Generally, the embodimentof FIG. 7 enhances this to include a third winding 725 for coupling thedata signal via a second neutral conductor 705 of power transmissioncable 700, and a fourth winding 745, inductively coupled to thirdwinding 725, for coupling the data signal via data port 760. The datasignal travels in a first path via first neutral conductors 702, firstwinding 720 and second winding 740, and in a second path via secondneutral conductor 705, third winding 725 and fourth winding 745. Thefirst path is in parallel with the second path.

FIG. 7 illustrates the use of all pairs of neutral wires, according tothe embodiment of FIG. 6D. Wire pairs 702, 705, 710 and 715 all performas transmission lines, in a manner similar to the selected pair 600 ofFIG. 6D. The segments of the neutral wires passing through the magneticflux of the cores act as windings 720, 725, 730 and 735, and drive theneutral wire pairs as transmission lines. Windings 740, 745, 750 and 755may be connected in parallel, as shown, or in any series or parallelcombination providing consistent phasing, to provide a data signal to aport 760. Since a central phase conductor 715 of power cable 700 isexposed to equal and oppositely phased flux from the coupling coils,phase conductor 715 does not affect the signal transmission.

Some of the advantage of the embodiment of FIG. 7 are (a) theinstallation of a coupler can be performed without selecting a pair ofneutral conductors, and therefore without identifying those conductorsat the far end of the segment (note that a phase inversion is possiblehere, but would not affect data flow, as modems can tolerate phaseinversion of the entire signal), (b) data transmission is possible, evenif cable 700 is damaged during its run, and some of the neutral wiresare accidentally grounded, (c) better cancellation of external fieldsand lower radiation, and (d) lower path loss over the cable segment.

FIG. 8 is a schematic of an embodiment of the present invention usingmultiple transmission lines with multiple sets of neutral wires. Thisembodiment utilizes any of the embodiments represented in FIGS. 6A-6D,but instead of a single signal path, it exploits a multiplicity ofneutral wire transmission lines 802, 805, 810, 815 to provide multipleindependent transmission channels. FIGS. 8 shows four transmissionchannels.

Similar to the embodiments of FIGS. 6A-6D, the embodiment of FIG. 8includes a coupler having a first winding 820 for coupling a data signalvia a first neutral conductor 802 of a power transmission cable 800 anda second winding 825, inductively coupled to first winding 820, forcoupling the data signal via a data port 830. Generally, the embodimentof FIG. 8 enhances this to include a third winding 835 for coupling asecond data signal via a second neutral conductor 805 of powertransmission cable 800, and a fourth winding 840, inductively coupled tothird winding 835, for coupling the second data signal to a second dataport 845.

Such a multiplicity may be exploited for achieving (a) full duplextransmission of data on one or more channels, (b) multipleunidirectional or bi-directional channels, thus increasing overallbandwidth, (c) redundant transmission of data to minimize errors, (d)implementing multi-wire interfaces that have separate clock, strobe anddata lines, and (e) use of one channel for supervisory commands, errornotification, or other data useful in network management.

For each of the embodiments shown in FIGS. 6A-6D, and for theenhancements shown in FIGS. 3-8, selection of one or two neutral wiresat one end of a cable implies that the same wires must be identified atthe distal end of the cable.

FIG. 9A is a schematic, and FIG. 9B is an illustration, of a system 900for identifying one of a plurality of wires of a power transmissioncable. System 900 includes a receiver 902 for sensing a signal from aselected neutral wire of the power transmission cable, and an indicator905 of a magnitude of the signal. The signal is applied to a selectedwire 925 at a first point 926 on the power transmission cable. Receiver902 senses the signal at a second point 927 on the power transmissioncable that is remote from the first point.

System 900 also includes a ferrite toroid 915 having a radial slot 920through which the selected neutral wire 925 is routed, and a winding 930that is wound around a portion of ferrite toroid 915 and connected to aninput 935 of receiver 902. The signal is inductively coupled from theselected neutral wire 925 via the ferrite toroid 915. The signal isapplied to the selected neutral wire 925 at first point 926 via aninductive coupler 924.

At the first cable end to be connected, the wire(s) are selected, and acoupler attached. FIG. 9A shows a pair of neutral wires being selected.The coupler is driven by a low power, high frequency oscillator,typically in the MHz range. This causes high frequency current to flowmost strongly in the wire(s) selected.

At the distal end, radio receiver 900 is tuned to the same frequency.This radio receiver is special in that it is equipped with a signalstrength meter 905 and manual or automatic gain control 910 foroptimizing the gain. In addition, the receiver's antenna comprises aferrite toroid 915 with a radial slot 920 slightly greater than thediameter of the neutral wire 925, and a coil wound on the toroid 915connected to the receiver's antenna input terminals 935. Preferably,toroid 915 is fixed-mounted onto the receiver case.

The installer holds the receiver so as to orient the slot to be in linewith and proximate to neutral conductor 925 and observes the reading onthe signal strength meter 905. The installer then moves the receivertangentially around the cable, sensing each wire in turn. The wire(s)producing the maximum reading on the signal strength meter will be thosedirectly excited at the other end of the cable.

Accordingly, a method for identifying one of a plurality of neutralwires of a power transmission cable, comprises the steps of (a) applyinga signal to a selected neutral wire, at a first point on the powertransmission cable, (b) sensing a relative magnitude of the signal oneach of the plurality of neutral wires at a second point on the powertransmission cable that is remote from the first point, and (c)identifying the selected neutral wire from the relative magnitudes. Theidentifying step identifies the selected neutral wire as the one of theplurality of neutral wires having a greatest relative magnitude. Theapplying step comprises inductively coupling the signal to the selectedneutral wire, and the sensing step comprises inductively coupling thesignal from the selected neutral wire.

Thus far, the present invention has been described in the context of acable with multiple, separate, mutually insulated neutral wires.However, many power distribution networks do not use cables withmutually insulated neutral wires, but rather have their neutral wires inthe form of a mesh or multiple wires connected together with conductingcopper tape. FIGS. 10A, 10B, 11A and 11B and their associateddescriptions, relate to an application of the present invention forother common medium voltage power girds, such as those carried onoverhead wires and those carried on pseudo-coaxial underground cableswith a single neutral conductor.

A coupler that avoids physical contact with a medium voltage phaseconductor is desirable in that such a coupler would not need towithstand steady state and surge voltages of the phase conductor, thussimplifying construction and reducing cost of the coupler. However, theuse of the currently proposed inductive coupler presupposes a circuitcontinuity through which current may flow, whilst the medium voltagecircuits may include either physically open circuits at their ends, orbe connected to transformer windings whose high impedance at radiofrequencies may approximate the effect of an open circuit termination.In accordance with the present invention, inductive couplers can be usedin a medium voltage data backhaul network when high frequencyterminations are added using capacitive-coupled ports at the ends of thecable, and, in a large distribution network, also at a one or moreintermediate positions. The phase conductors of underground powertransmission lines can be used as data transmission lines when they areequipped with load terminations effective at the high frequencies usedfor communications for coupling data signals to and from thetransmission lines.

In power distribution systems, the medium voltage grid is attached todevices that present an impedance much higher than the cable'scharacteristic impedance to signals at high frequencies. Such deviceseffectively appear as open circuits to high frequency signals. Couplingmodulated data packets onto such an open-circuited cable would result ina large fraction of a coupled wave being reflected from the ends of thecable, and possibly being interpreted by data receivers as new packets.A further undesirable feature of such reflections would be to misleadthe data receivers into concluding that new packets are occupying thecable, and “carrier sense” types of shared networks would suffer a lossof available transmission time.

For cables and wires with significant high frequency losses, thesereflections would quickly dissipate, and not cause problems. However,for both overhead lines and some underground pseudo-coaxial lines, thelosses are low, and strong reflected signals may interfere with thedirect signals.

FIGS. 10A and 10B are schematics of portions of a data communicationnetwork implemented over a power distribution system, where the data iscarried on a phase conductor of the power distribution system. Thepresent invention uses a combination of inductive and capacitivecouplers. As explained below, the network includes (a) an inductivecoupler for coupling a data signal via the phase conductor, and having adata port for further coupling of the data signal, and (b) a capacitivecoupler, connected between the phase conductor and ground, proximate toan end of the power transmission cable, for absorbing reflections of thedata signal and optionally serving as a data port for coupling of thedata signal.

Inductive couplers 1002 are used at intermediate nodes 1005 proximate toa distribution transformer 1010. Each inductive coupler 1002 provides aport 1015 for connection to a modem (not shown) over a low voltagenetwork being powered from the secondary of each distributiontransformer 1010. Capacitive couplers 1020 are connected between an endof a wire or cable and a local ground, to both absorb reflections andprovide signal coupling nodes 1025. That is, a signal coupling node 1025is located between a capacitive coupler 1020 and ground, for couplingthe data signal between the phase conductor and for providing anotherdata port for the data signal.

The “end of the wire or cable” includes a point 1018 where power is fedinto the cable from a high voltage to medium voltage transformer. Inloop topologies, the cable returns to this location but reaches a deadend. Capacitive couplers 1020 are included at such “dead ends”. Should aT-branch 1030 produce a stub 1035 in the power network, a capacitivecoupler 1020 is used to terminate the distal end of stub 1035.

FIG. 11A is an illustration of an embodiment of an inductive coupler1102 for coupling data via a phase conductor, in accordance with thepresent invention. FIG. 11B is a schematic representation of theembodiment shown in FIG. 11A.

An inductive coupler 1102 includes a first winding 1104 for coupling thedata signal via a phase conductor 1110, and a second winding 1115,inductively coupled to first winding 1104, for coupling the data signalvia a data port 1145. Inductive coupler 1102 includes a core 1105through which phase conductor 1110 is routed. This configuration ofphase conductor 1110 through core 1105 serves as first winding 1104,i.e., a winding of a single turn. Second winding 1115 is wound around aportion of core 1105.

Inductive coupler 1102 is a current transformer in which core 1105 isplaced over a segment of phase conductor 1110. Inductive coupler 1102can also be used with an underground cable by placing core 1105 over asegment of an underground cable that is not also covered by a neutralconductor sheath, with the power cable phase wire passing through core1105 as a one-turn winding.

Core 1105 is made of ferrite or other soft magnetic material withsubstantial permeability and relatively low loss over the frequencyrange required for the modulated data. Core 1105 has an air gap 1120sufficient to allow operation of the inductive coupler 1102 withoutsaturation, even when current through phase conductor 1110 is as high asthe maximum current for which conductor 1110 is rated, e.g. 200 ampsrms.

Inductive coupler 1102 has a primary magnetization inductance sufficientto present appreciable high frequency impedance to a modem transmitterover a relevant frequency range, but negligible impedance at powerdistribution frequencies. Inductive coupler 1102 has both a leakageinductance and a reflected-primary impedance much lower than thecharacteristic impedance of the transmission line of which phaseconductor 1110 is a component, over the relevant frequency range.

Inductive coupler 1102 has a high voltage capacitor 1125 in series withsecond winding 1115 and data port 1145, and connected to a low voltageoutput, i.e. power line output, of a distribution transformer 1130, toprevent second winding 1115 from short circuiting a low voltage powercircuit 1135. Thus capacitor 1125 couples a data signal between secondwinding 1115 and the power line output.

Inductive coupler 1102 also has a surge protector 1140 connected inparallel with second winding 1115, to protect the low voltage circuit1135, and any electronic communications equipment attached to thereto,from being affected by a high amplitude pulse that might appear on phaseconductor 1110 and be coupled by inductive coupler 1102 onto the lowvoltage lines.

Note that while only one LV phase line 1150 and LV neutral line 1155 areconnected to coupler 1102, the other phase line 1160 will receive aslightly attenuated signal via capacitive and inductive coupling, overthe length of the LV drop lines.

An important consideration, and a desirable objective, is a minimizationof electromagnetic radiation from the wires and cables used fortransmission of data. These lines could radiate electromagneticinterference, even if buried a few feet underground. Spurious resonancesmight also prevent transmission over certain narrow frequency bands.

One or more techniques should be employed to minimize radiation,tolerate resonances, and provide a robust and reliable data channel.Options for minimizing radiation include:

(A) Using spread spectrum modulation in the modems connecting to andfrom the medium voltage grid. Spread spectrum modulation employs arelatively low spectral power density (e.g. −55 dBm/Hz).

(B) Minimizing the power level of the modulated data. The power levelshould to be high enough to overcome any noise on the line, and anyself-generated equipment noise, e.g., internal noise, amplifier noise,etc. By exploiting the relative isolation of the medium voltage linefrom the noisy low voltage and high voltage grids, line noise can beminimized. This can be accomplished by placing back-to-back modems ateach inductive coupler. Back-to-back modems are for the purpose ofregenerating a bit stream and remodulating the data transmission over anadditional medium.

FIG. 12 is a schematic of a portion of a network having back-to-backmodems at an inductive coupler. A first modem 1202 has a first port 1225coupled to a data port of a second winding of an inductive coupler 1102for sending and receiving a modulated data signal, and a second port1210 for further coupling of the digital data. A second modem 1205 has afirst digital data port 1230 coupled to the second port 1210 of firstmodem 1202, and a second port 1235 for further coupling of the modulateddata signal. Optionally, a router 1220 may be interposed between firstmodem 1202 and second modem 1205.

The advantages of the above arrangement are:

A) The noise of the LV grid does not reach the MV grid. Isolation canfurther be enhanced by optical isolators in series with the dataconnection 1210.

B) A spread spectrum or other modem, which uses different technology orparameters than the MV modem, can be optimized for LV grids. Theinductive couplers introduce additional series impedance at the couplingnodes that is small relative to the wire or cable's characteristicimpedance, thus minimizing both reflections and power absorption. Inthis case, the modulated data may traverse a large number ofintermediate nodes successfully. Preferably, the magnetization andleakage inductances are small enough to low voltage and high grids, linenoise can be minimized. This can be accomplished by placing back-to-backmodems at each inductive coupler. Back-to-back modems are for thepurpose of requesting a bit stream and remodulating the datatransmission over an additional medium.

FIG. 12 is a schematic of a portion of a network having back-to-backmodems at an inductive coupler. A first modem 1202 has a first port 1225coupled to a data port of a second winding of an inductive coupler 1102for sending and receiving a modulated data signal, and a second port1210 for further coupling of the digital data. A second modem 1205 has afirst digital data port 1230 coupled to the second port 1210 of firstmodem 1202, and a second port 1235 for further coupling of the modulateddata signal. Optionally, a router 1220 may be interposed between firstmodem 1202 and second modem 1205.

The advantages of the above arrangement are:

A) The noise of the LV grid does not reach the MV grid. Isolation canfurther be enhanced by optical isolators in series with the dataconnection 1210.

B) A spread spectrum or the other modem, which uses different technologyor parameters than the MV modem, can be optimized for LV grids. Theinductive couplers introduce additional series impedance at the couplingnodes that is small relative to the wire or cable's characteristicimpedance, thus minimizing both reflections and power absorption. Inthis case, the modulated data may traverse a large number ofintermediate nodes successfully. Preferably, the magnetization andleakage inductances are small enough to minimize impedance disturbancebut large enough to provide sufficient coupling. Implied here is anintentional impedance mismatch between the modem and the impedancepresented by the coupler.C) Routers and other networking equipment 1220 can be employed formediating between the home and external network.

One parameter at issue for minimizing radiation is an attenuation ofsignal level in a direction between line and coupler, as the signallevel on the medium voltage power line must be strong enough to overcomethis attenuation. Attenuation in the direction between coupler and linemay be easily overcome without additional radiation by applying morepower to the coupler driving the line so as to establish the maximumpermissible transmitted power level consistent with compliance tomaximum allowed radiation levels.

For example, if each coupler is designed for a 10 dB coupling loss, thenthe transmitted power may be increased by 10 dB to compensate, and onlythe second coupler's 10 dB is deducted from the modem's loss budget.

FIG. 13 is a schematic of a technique for passively coupling modulateddata between segments of a power grid, in accordance with the presentinvention. FIG. 13 shows a data communication network 1300 implementedover a power distribution system having a first segment 1302 with afirst neutral conductor 1320, and a second segment 1303 with a secondneutral conductor 1330. Network 1300 includes a first coupler 1306 forinductively coupling a data signal via first neutral conductor 1320, andhaving a data port 1335 for further coupling of the data signal, and asecond coupler 1307 having a data port 1340 coupled to data port 1335 offirst inductive coupler 1306, and for inductively coupling the datasignal via second neutral conductor 1330.

First segment 1302 includes a first power distribution cable 1315 on afirst side of a power distribution transformer 1345. Second segment 1303includes a second power distribution cable 1325 on a second side ofpower distribution transformer 1345. Power distribution transformer 1345has an output to power line 1350. Network 1300 further comprises acapacitor 1310 between data port 1335 of first inductive coupler 1306and output power line 1350, for coupling the data signal to output powerline 1350.

Each transformer-to-transformer segment becomes a separate link in amulti-link chain. A coupler is attached to each cable termination, thusrequiring two couplers per transformer, except for the last transformeron a dead end segment.

Passive chaining of segments is achieved by connecting the data ports1335 and 1340 of the two couplers on either side of a transformer toeach other. A passive connection to the communications devices attachedto LV line 1350 is made through series coupling capacitors 1310. Similarmodems would be attached at both the networks feed point, such as thepower substation, and at low voltage outlets at the users' premises.

FIG. 14 is a schematic of a technique for coupling modulated databetween segments of a power grid using back-to-back modems. FIG. 14shows a data communication network 1400 implemented over a powerdistribution system having a first segment 1402 with a first neutralconductor 1420, and a second segment 1403 with a second neutralconductor 1430. Network 1400 includes a first coupler 1406 forinductively coupling a data signal via first neutral conductor 1420, andhaving a data port 1435 for further coupling of the data signal, and asecond coupler 1407 having a data port 1440 coupled to data port 1435 offirst inductive coupler 1406, and for inductively coupling the datasignal via second neutral conductor 1430.

A first modem 1460 includes a first port for modulated data signals 1465coupled to data port 1435 of first coupler 1406, and having a secondport for digital data 1470 for further coupling of the data signal. Asecond modem 1480 has a first port for digital data 1475 coupled tosecond port 1470 of first modem 1460, and a second port 1485 for furthercoupling of the modulated data signal.

The power distribution system includes a power distribution transformer1445 having an output power line 1450. Network 1400 further comprisescapacitors 1410 between second port 1485 of second modem 1480 and outputpower line 1450, for coupling the modulated data signal to output powerline 1450.

A medium voltage cable may include a long cable segment, such as from asubstation to the first distribution transformer in a loop. For ease ofinstallation and service, the long section may be segmented, with accessmanholes at each node. At these points, the cable segments might beterminated in medium voltage connectors (for center conductor), alongwith neutral wire collector rings that are grounded. This introduces adiscontinuity in the data transmission line, which is carried on one ormore neutral wires. To bypass this discontinuity, a pair of couplers canbe installed, one on either side of the ground, with their primariesconnected to each other, creating a bridging connection.

The present invention also provides for implementing a datacommunication network using a phase conductor across segments of a powerdistribution system.

FIG. 15 is a schematic showing several techniques for coupling data to aphase conductor of a power distribution system in an implementation of adata communication network 1500, in accordance with the presentinvention.

A capacitive coupler is placed on overhead lines fed by an HV-MVstep-down transformer. The transformer secondary impedance is of thesame order of magnitude as that of overhead lines or larger. Aterminator-coupler, e.g., a capacitive coupler with a data port, may beused here that both (a) is used to couple a modem to the line, and (b)terminates the line with a resistance approximately equal to thecharacteristic impedance of the power transmission cable (as a modem ordummy resistor impedance is reflected through its transformer).Accordingly, FIG. 15 shows that the power distribution system includes asubstation HV-MV voltage step-down transformer 1502. A capacitivecoupler 1535, i.e., a terminator-coupler, is located proximate to asecondary winding of voltage step-down transformer 1502. A component,such as modem 1525, has an impedance that when reflected throughcapacitive coupler 1535 is approximately equal to a characteristicimpedance of the power transmission cable.

In systems such as in Japan, where the custom is to run a very lowimpedance coaxial underground cables for lengths up to hundreds ofmeters to the beginning of an overhead grid, the preferred location forinductive couplers is at the overhead side of the underground-overheadtransition point. Here, the low impedance of the underground cable actslike a short circuit at the end of the overhead line, and a closedcurrent loop is formed. Thus, the power distribution system includes atransition 1545 between an overhead cable 1515, 1516 and an undergroundcable 1510, in which underground cable 1510 has a characteristicimpedance that is much lower than that of overhead cable 1515. One ormore inductive couplers 1540, 1541 are located on overhead cable 1515,1516, proximate to transition 1545.

The placement of inductive couplers 1540, 1541 on the three phaseoverhead cable 1515, 1516 may be done symmetrically with each member ofa coupler pair driven with oppositely phased current. Such a drive willsubstantially cancel the far field electromagnetic radiation, and easecompliance with any regulatory standards. Accordingly, network 1500 mayinclude a pair of inductive couplers 1540, 1541 such that a firstinductive coupler, e.g., 1540, of the pair induces a first current inthe phase conductor, e.g., 1515, in a first direction, and a secondinductive coupler, e.g., 1541, of the pair induces a second current inthe second phase conductor, e.g., 1516, in a direction opposite of thefirst current.

Alternatively, a single phase may be driven, with equal and oppositecurrents being induced in the other phases, at a distance exceeding onewavelength from an inductive coupler, again canceling much of the farfield radiation. For example, one inductive coupler 1540 may be used,and transmission line induction effects may be relied on to balance thecurrents, after one wavelength down the line.

Inductive couplers may also be placed on the lines feeding adistribution transformer primary, since the transformer primaryimpedance of some types of distribution transformers may be of the sameorder of magnitude as that of the overhead lines, and a closed loop isformed. Since this loop carries relatively low power-frequency currents,typically in the 2-8 Ampere range, there is little tendency for coresaturation, and coupler cores may be built with little or no air gap. Asshown in FIG. 15, an inductive coupler 1550 is located on a line thatfeeds a primary winding 1555 of a distribution transformer of the powerdistribution system.

Since the magnitude of the circuit impedance seen by the inductivecoupler 1550 may be as high as hundreds of ohms, and modems 1560 alongthe length of the transmission line attached to inductive coupler 1550would typically have a 50 ohm impedance, there may be a substantialimpedance mismatch.

As shown in FIG. 15, power distribution system 1500 may include a PLCcapacitor and/or a power factor correction capacitor, e.g., capacitor1565, between a phase conductor, e.g., 1516 and ground. Capacitor 1565may have an impedance lower than that of the power transmission cable1516. PLC and power factor capacitors may have a high RF impedance, inwhich case they will not significantly disturb HF signals passing overthe power grid. For those devices having an RF impedance whose magnitudeis of the same order of magnitude or lower than the characteristicimpedance of the power line, such as capacitor 1565, a series choke 1570may be inserted in series with capacitor 1565. Series choke 1570 maycomprise an existing lead wire 1575 to capacitor 1565 by placing one ormore snap-on split magnetic cores over lead wire 1575.

The power-frequency current is relatively low, so core saturation willnot be a problem. The micro-Henry magnitude of these chokes' inductiveimpedances will not affect the capacitors' power frequency functioning.Lossy cores may also be used, for they simply increase the highfrequency impedance of the choke, and add to the isolation of thecapacitor.

The effects of transmission line reflections must be considered as theyproduce echoes that might introduce errors in the data stream. Spreadspectrum modulation is the most likely candidate for such echo-ladentransmission, as it is tolerant of narrow band frequency absorption andnarrow-band noise, and minimizes emitted electromagnetic radiation dueto its low spectral power density. For spread spectrum modems, reflectedintra-packet signals that are 6-10 dB or more below the direct signallevel will not affect the data reception. Intra-packet reflected signalsare defined as reflections that arrive during the direct reception ofthe original packet.

Impedance disturbances on the power lines may be caused by (a)distribution transformers, with or without the addition of inductivecoupler impedance, (b) line terminations, which are typically designedto be fairly well matched to line impedance, (c) T-branches, and (d) PLCor power factor correction capacitors. The reflection coefficient ofthese impedance discontinuities will generally not exceed 0.5, and thereflected signal is subject to outbound and return loss of the linesthemselves, i.e., absorptive loss and radiation loss, so it is expectedthat the amplitude of reflected signals will be weaker than directsignals by more than 6-10 dB. Thus, the reflected signals that arriveduring a data packet will appear as low amplitude noise, and will notprevent the intended data signals from being correctly received.

For couplers placed at low impedance feed points to high impedancelines, such as transition 1545, the loss and reflections due toimpedance mismatch are not desirable. Since the very heavy power wirescannot be wound around the coupler core, the secondary can have no morethan one turn, and the primary can have no less than one turn.Therefore, the impedance reflected onto the power lines will be equal tothe modem impedance, one quarter of that, or less, depending on theturns ratio. For modems with 50-ohm terminal impedance, this reflectedimpedance is much lower than their characteristic impedance. Onesolution to improving impedance match is to build modems with an outputimpedance of a few hundred ohms.

Another solution is to connect a phase-antiphase pair of couplers withtheir primaries in parallel. The secondaries (MV lines) are necessarilyin series. Thus, a 50-ohm modem impedance is transformed into a 100 ohmreflected impedance by the phase-antiphase inductive coupler pair. Thisprinciple can be carried further, by using multiple couplers with theirprimaries paralleled, achieving a series connection of transformer(coupler) windings on the power line side, and a parallel connection onthe modem side.

For example, FIG. 15 shows a first inductive coupler 1540 and a secondinductive coupler 1541. First inductive coupler 1540 induces a firstcurrent in a first direction in phase conductor 1515 via a first winding1540A, and second inductive coupler 1541 induces a second current in theopposite direction in phase conductor 1516 via a second winding 1541A.First winding 1540A and second winding 1541A are in parallel with oneanother. In FIG. 15, first winding 1540A and second winding 1541A aremarked with dots to show this phase relationship.

The inductive coupler at the overhead feed point must be designed towithstand the effects of the total feed current, which may reachhundreds of Amperes. Since even a one-turn coil carrying such currentwill saturate the core of currently available magnetic materialsappropriate for high frequency operation, this “main line” coupler mustgenerally include an air gap in its magnetic circuit. To achievesufficient magnetization inductance, such couplers will need amultiplicity of cores forming the equivalent of one core that is verythick in the direction of the power wire.

FIGS. 16A-16C are schematics representing several uses of capacitivecouplers in a communication network implemented over a powerdistribution system. These capacitive couplers are used at nodes in thenetwork where inductive couplers might not be effective, e.g., at pointswhere there is an effective open circuit to RF current.

A capacitive coupler 1020, such as used in FIGS. 10A and 10B, is shownin FIG. 16A, marked there as capacitive coupler 1600. Capacitive coupler1600 should be capable of continuously withstanding the working voltagesupplied by the phase conductor and a series of BIL pulses, e.g., 125 kVfor a 15 kV working voltage, as per IEEE Specification 386. Capacitivecoupler 1600 should also be constructed so as to eliminate coronabreakdown as per the above specification.

Capacitive coupler 1600 connects to the MV lines via high voltagecapacitors 1620, e.g., 10 nF, whose impedance at the lowest relevantfrequency is a fraction of the characteristic impedance of the powertransmission cable. Optionally, capacitive coupler 1600 may include asafety fuse 1625 in series with capacitor 1620, to avoid faulting themedium voltage line should in case of a short-circuit.

High resistance bleeder resistors 1605 are connected in parallel witheach capacitor 1620 to discharge them when they are not connected toenergized circuits. Charged capacitors would be a hazard to personnel.To further isolate the data port 1630 from the MV lines, a highfrequency isolation transformer 1615 is used, with an optional non-unityturns ratio, if needed, for impedance transformation.

To protect devices that are connected to data port 1630, a surgeprotector 1632, such as an metal oxide varistor (MOV) may be connectedacross the terminals of data port 1630 to limit the amplitude of pulsesthat might otherwise be coupled from the MV lines to the devices.

Preferably, in the network in which the capacitor is installed, oneterminal of capacitive coupler 1600 is connected to a medium voltagephase line, and the other terminal connected to neutral (for singlephase lines) or to a second phase line (for multiphase lines).

When used to terminate a dead end of a transmission line, capacitivecoupler 1600 may be used, together with a termination resistor 1635,connected to data port 1630, to match the power transmission cable'scharacteristic impedance.

FIG. 16B illustrates the use of capacitive coupler 1600 for coupling amodem 1636 onto a dead end of a power transmission cable. Modem 1636 isconnected to data port 1630.

FIG. 16C is a schematic of an arrangement of capacitive couplers formaintaining continuity of a data signal across a grid segmentationswitch. FIG. 16C shows a power distribution system having a phaseconductor with a first segment 1601 on a first side of a switch 1602 anda second segment 1603 on a second side of switch 1602. A firstcapacitive coupler 1650 couples a data signal via first segment 1601,and has a data port 1635 for further coupling of the data signal. Asecond capacitive coupler 1660 has a data port 1665 coupled to data port1635 of first capacitive coupler 1650, and couples the data signal viasecond segment 1603. Thus, a transmission of the data signal betweenfirst segment 1601 and second segment 1603 is maintained when switch1602 is opened.

The present invention employs a variety of network protocols to extendphysical range and improve reliability. After passing through inductivecouplers and encountering impedance mismatches, tee junctions, andradiation loss, the amplitude of the signal available to the modem'sreceiver may become very weak. Whether this weakness is relative to amodem's internal noise or to ambient electrical noise on the mediumvoltage lines, there will be a physical point beyond which the signalcannot be detected and demodulated into data with an acceptably lowerror rate.

Bi-directional modems may be added to regenerate and strengthen thesignal if high impedance chokes are also used to isolate the mediumvoltage grid into independent segments.

The data communication network can employ communication protocols thatinclude the passing of data tokens from node to node. At each node, thetoken, which provides signaling or control, or includes a data packet aspayload, is stored, interpreted, and routed to either the modem's localdata user or else to the next node on the network. The time required tostore, interpret and retransmit a token will reduce considerably theeffective net data rate of such a network, if each node is alwayson-line.

In accordance with the present invention, only certain nodes areprogrammed to be active at any given moment, namely the node to whichthe token is addressed and a minimum fixed subset of nodes distributedalong the network that are required to maintain a minimum signalamplitude for all points on the network. When this subset of nodes isactive, there will be an advantageous tradeoff of time delay and reducednet data rate in return for increased physical range and improved errorrate.

The determination of identity of the permanently active node members canbe achieved by manual measurements of the attenuation between all nodesof the medium voltage network. Preferably, the modems are equipped withcircuits that measure the voltage amplitude and/or signal to noiseratio, and are interrogated by a network media access control layer. Thenodes should also be programmed to accept a command that keeps them in apermanently active relaying mode, even for tokens or packets that arenot addressed to them.

An algorithm can then be implemented that determines which nodes shouldbe set permanently active, and issues a stream of commands to all nodesto set the appropriate nodes permanently active. The algorithm is runeach time the configuration of the medium voltage grid is changed, butthis is a relatively rare event.

Closely spaced nodes will enjoy a data rate equal to the maximum networkrate while more distant nodes will still be serviced by reliable, lowerror rate service, albeit at a lower data rate. In principle, it isclaimed that the described arrangement removes all distance limits frommedium voltage communications.

The transmission line formed by the selected conductor and its neighborsis inherently a wide bandwidth, low loss, and low dispersion medium. Foroverhead lines, the losses would be due to skin effect and radiation,the latter being relatively ineffective since the lines are not resonantat most frequencies. For underground lines, the losses would be due toskin effect and the insulation's lossiness, e.g., outer layer of plasticand inner layer of semiconducting material.

The present invention produces a low electromagnetic emission and has alow susceptibility to external noise, especially when used with spreadspectrum techniques. Power levels may also be kept low, because of lowcable-to-coupler loss. Susceptibility to external noise sources would beproportional to radiation, with the modes having the lowestelectromagnetic interference (EMI) also being those most resistant toexternal noise reception, based on the reciprocity principle.

For single-ended mode (see FIG. 2A), the two neighbors of the selectedconductor act in anti-phase to the central conductor for both electricaland magnetic radiation modes. An observer at distance would seesubstantial cancellation of fields.

For balanced modes, there would be both far-field cancellation and ashielding affect of the grounded neighbors. For transformer termination(See FIG. 2B), the coupling loss would be lowest, and drive power levelscould be kept relatively low, giving the lowest EMI levels. For choketermination, drive power levels would be slightly higher.

If the modems act as repeaters, then drive power levels can be held tothe minimum required for a single segment, further reducing radiation.

A data communication network in accordance with the present inventionoffers a capacity for very high data rates, e.g., exceeding 10 Mbps. Thecouplers are all magnetic and electrostatic devices with bandwidths thatcan reach at least tens of megahertz if high frequency magnetic anddielectric materials are used. Transmission lines that are not too lossyand that have minimal dispersion could conduct frequencies exceeding 20MHz. Such frequencies could be used for modems using various modulationschemes, and even at one bit per Hertz, would yield high data rates.

Baseband signaling can also be employed if the data coding eliminateslong strings of all-1's and all-0's. With inter-segment connections thatinclude regeneration (repeaters), the bandwidth would be much largerthan obtainable with passive linking of segments.

The couplers of the present invention can be installed with little or nointerruption of power service to customers. Installation can also beaccomplished without exposure to high voltages. Employing glovedlinemen, the authorities may allow placing an inductive coupler around acable, while the cable is in service. Even if the authorities insistthat workmen not work on energized cables, the loop architecture ofneighborhood medium voltage grids allows disconnecting a single cablesegment, without interrupting service to customers. For the relativelyfew capacitive couplers, a single short power outage might be needed.

The present invention permits continued operation of a datacommunication network even during power outage. Operation continues evenduring interruptions in medium voltage power.

The present invention poses little or no impact on reliability ofelectrical grid. Inductive couplers have no fault modes that wouldaffect power flow. The few capacitive couplers, with their fuses, wouldalso not cause a line fault.

For the embodiments of FIGS. 2A and 2B, winding the coupler with athicker wire would preclude its failure due to over-current, and usingindustry-standard connections between the selected neutral and thecoupler should minimize connection failure. Were an open circuit tooccur, it leaves (N−1)/N of the current-carrying capacity intact, or87.5% in the case discussed. Since the cable usually runs at much belowits 200 A capacity, such a failure should have no effect.

Short circuit of the coupler would impact data communications, but suchwould merely restore the neutral conductor to its original state.Accordingly, the power grid would not be adversely impacted.

Short circuit of the neutral or any other part of the coupler to groundwould have not effect on the MV line, as its neutral is proximatelyconnected to the ground rod. Failure of the magnetic circuit, open,short or saturation, would have no effect on the supply of electricpower or the safety of system.

The couplers use only passive components, implying a virtually unlimitedlife of service. The inductive coupler can be any suitable transformeror inductor.

In the passive implementation, the inductive couplers use only passivecomponents, e.g. wires wound around magnetic cores, and these have nowear mechanisms. The capacitive couplers also have no wear mechanisms.

The passive construction, and ease of installation of the inductivecouplers provide a low cost solution to the problem of coupling tomedium voltage power distribution lines and utilizing them a backhauldata channels. Installation time should be less than 15 minutes for thepredominant inductive, and installation costs minimal.

There is a clear advantage of the embodiments that use neutral lines, ascompared to capacitive bypass couplers that employ the medium voltageconductors to carry data. The latter make contact with the mediumvoltage line at least once each transformer, and must withstand fullfault voltages. For example, a Coupler for a 15 kV rms phase-to-groundcable must be tested for 125 kV BIL. This makes the capacitive couplervery bulky and expensive, and adds many more potential points of failureto the system.

It should be understood that various alternatives and modificationscould be devised by those skilled in the art. The present invention isintended to embrace all such alternatives, modifications and variancesthat fall within the scope of the appended claims.

1. A coupler for a data signal, comprising: a core through which a phaseconductor of a power distribution system can be routed to serve as afirst winding; and a second winding wound around a portion of said core,wherein said first winding has an impedance that is negligible at apower distribution frequency of said power distribution system, andwherein said data signal is inductively coupled between said phaseconductor and said second winding via said core.
 2. The coupler of claim1, wherein said core has an air gap.
 3. The coupler of claim 1, furthercomprising a capacitor in series with said second winding.
 4. Thecoupler of claim 1, further comprising a surge protector connected inparallel with said second winding.
 5. The coupler of claim 1, whereinsaid coupler comprises a radio frequency (RF) transformer.
 6. Thecoupler of claim 1, wherein said data signal is bi-directionally coupledbetween said phase conductor and said second winding via said core. 7.The coupler of claim 1, wherein said data signal is carried by a carrierfrequency in a range of about 1 MHz to about 50 MHz.
 8. The coupler ofclaim 1, wherein said data signal is carried by a carrier frequencygreater than or equal to about 10 MHz.
 9. The coupler of claim 1,wherein said phase conductor carries a power signal having a voltage ina range of about 4 kV to about 66 kV.
 10. The coupler of claim 1,wherein said data signal is carried by a spread spectrum modulatedcarrier.
 11. The coupler of claim 1, wherein said coupler presents areflected impedance at a frequency of a carrier of said data signal,wherein said phase conductor presents a characteristic impedance at saidfrequency of said carrier, and wherein said reflected impedance is lessthan said characteristic impedance.
 12. The coupler of claim 1, whereinsaid coupler has a leakage inductance with an impedance at a frequencyof a carrier of said data signal, wherein said phase conductor presentsa characteristic impedance at said frequency of said carrier, andwherein said impedance of said leakage inductance is less than saidcharacteristic impedance.
 13. The coupler of claim 1, wherein said phaseconductor passing through said core has an inductance sufficient topresent appreciable high frequency impedance to a modem transmitter at afrequency of a carrier of said data signal.
 14. A data communicationnetwork implemented over a power distribution system having a powertransmission cable with a phase conductor, said data communicationnetwork comprising: a first coupler having a core through which saidphase conductor can be routed to serve as a first winding, and a secondwinding wound around a portion of said core, wherein said first windinghas an impedance that is negligible at a power distribution frequency ofsaid power distribution system, and wherein said data signal isinductively coupled between said phase conductor and said second windingvia said core; and a second coupler for coupling said data signal viasaid phase conductor, wherein said data signal is communicated betweensaid first coupler and said second coupler via said phase conductor. 15.The data communication network of claim 14, wherein said phase conductoris part of a medium voltage grid of said power distribution system. 16.The data communication network of claim 14, further comprising: a firstmodem having a first port for coupling said data signal via said firstcoupler, and having a second port for further coupling of said datasignal; a router having a first port for coupling said data signal viasaid second port of said first modem and having a second port forfurther coupling of said data signal; and a second modem having a firstport coupled to said second port of said router, and having a secondport for coupling said data signal via said second coupler.
 17. The datacommunications network of claim 14, wherein said data signal isbi-directionally coupled between said phase conductor and said secondwinding via said core.
 18. The data communications network of claim 14,wherein said data signal is carried by a carrier frequency in a range ofabout 1 MHz to about 50 MHz.
 19. The data communications network ofclaim 14, wherein said data signal is carried by a carrier frequencygreater than or equal to about 10 MHz.
 20. The data communicationsnetwork of claim 14, wherein said phase conductor carries a power signalhaving a voltage in a range of about 4 kV to about 66 kV.
 21. The datacommunications network of claim 14, wherein said data signal is carriedby a spread spectrum modulated carrier.
 22. A coupler for a data signal,comprising: a core through which a phase conductor of a powerdistribution system can be routed to serve as a first winding, whereinsaid phase conductor carries a power signal having a voltage in a rangeof about 4 kV to about 66 kV; and a second winding wound around aportion of said core, wherein said first winding has an impedance thatis negligible at a power distribution frequency of said powerdistribution system, and wherein said data signal is carried by a spreadspectrum modulated carrier and inductively coupled between said phaseconductor and said second winding via said core.
 23. A method,comprising: routing a phase conductor of a power distribution systemthrough a core of a coupler to serve as a first winding, wherein saidfirst winding has an impedance that is negligible at a powerdistribution frequency of said power distribution system; andinductively coupling a data signal carried by a spread spectrummodulated carrier, via said core, between said phase conductor and asecond winding wound around a portion of said core.
 24. The method ofclaim 23, wherein said data signal is bi-directionally coupled betweensaid phase conductor and said second winding via said core.
 25. Themethod of claim 23, wherein said carrier is in a frequency range ofabout 1 MHz to about 50 MHz.
 26. The method of claim 23, wherein saidcarrier is at a frequency greater than or equal to about 10 MHz.
 27. Themethod of claim 23, wherein said phase conductor carries a power signalhaving a voltage in a range of about 4 kV to about 66 kV.